Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility grid is monitored such that, it the commercial electrical power from the utility grid fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical power generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator. As is known, most residential electric equipment in the United States is designed to be used in connection with electrical power having a fixed frequency, namely, sixty (60) hertz (Hz).
Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. The frequency of the electrical power output from the alternator is a function of the rotational speed of the driving engine. For example, a two-pole generator outputs sixty (60) hertz (Hz) when the engine is rotating at 3600 revolutions per minute (rpm) but only outputs forty-five (45) Hz when the engine is rotating at 2700 rpm. When the generator includes a single-phase alternator, the alternator has a pulsating torque that ranges from zero (0) to the maximum value corresponding to the generator design.
Improvements in power conversion have led to an increase in distributed power generation. Individual businesses and residences may include, for example, a photovoltaic (PV) array to generate at least a portion of the power required by that business or residence. In addition, a power storage device, such as a battery, may be included to store excess energy generated during periods of peak production and deliver the energy to the business or residence during periods of low production. The power is typically transferred from the alternate energy source or from the battery to the business or residence via an inverter. The inverter is able to generate a voltage waveform synchronized to the utility grid. If the utility grid fails and the business or residence also has an electrical generator, it is desirable to utilize the electrical generator in combination with the alternate energy source or battery.
If two alternating current (AC) power sources arc to be connected in parallel, the AC output voltages must be synchronized otherwise the instantaneous difference in voltage potential may result in current transferred between the two voltage sources. However, the pulsating torque produced by a single-phase generator may result in some fluctuation in frequency of the power output by the generator. Variations in the load applied to the generator may also cause fluctuation in the frequency of the power output by the generator. In contrast, the power output by the inverter is typically regulated at a constant frequency. If the generator is connected in parallel to the inverter, the variable frequency of the output power from the generator produces voltage waveforms from the generator that are out of phase with the constant frequency voltage waveforms output from the inverter.
Therefore, it is a primary object and feature of the present invention to provide a system and method for connecting a single-phase generator in parallel with an inverter.
It is a further object and feature of the present invention to provide a system and method which permits the inverter to run independently of the single-phase generator in a first mode and in parallel with the single-phase generator in a second mode.
In accordance with one embodiment of the present invention, a method for controlling operation of an inverter having an output configured to he connected in parallel with an output of a single-phase generator is disclosed. An input signal is received at the inverter, where the input signal corresponds to a measured or estimated angular position of a rotor in the single-phase generator. The inverter executes a modulation routine to generate an AC voltage at the output of the inverter. The modulation routine receives the input signal corresponding to the angular position of the rotor and synchronizes a phase angle of the AC voltage to the angular position of the rotor. It is contemplated that the signal corresponding to the angular position of the rotor may be generated, for example, by an angular position sensor connected to the rotor, a measured voltage or current output from the single-phase generator, an estimate based on generator parameters, or one or more firing signals corresponding to ignition of fuel in each cylinder of the engine driving the single-phase generator.
According to another aspect of the invention, the inverter may be configured to execute in a first operating mode and in a second operating mode. During the first operating mode, the inverter executes the modulation routine independent of the input signal, and during the second operating mode, the inverter executes the modulation routine to synchronize the phase angle of the AC voltage to the angular position of the rotor. A switch may be used to selectively connect and disconnect the output of the inverter with the output of the single-phase generator. In the first operating mode, the inverter operates with the switch disconnecting the output of the inverter from the output of the single-phase generator. The inverter then receives a second input signal at the inverter corresponding to an indication that the single-phase generator is generating voltage at a desired magnitude and frequency and transitions from the first operating mode to the second operating mode. In the second operating mode, the inverter generates a control signal for the switch to connect the output of the inverter with the output of the single-phase generator.
According to another embodiment of the invention, a system for synchronizing a voltage output by an inverter with a voltage generated by a single-phase generator is disclosed. The single-phase generator includes an engine having a shaft rotated by operation of the engine and an alternator. The alternator includes a rotor operatively coupled to the shaft and a stator having a winding. The rotor is rotated by rotation of the shaft from the engine and the voltage is generated on the winding as a function of the rotation of the rotor. The inverter includes an input configured to receive an input signal corresponding to an angular position of the rotor and a processor configured to execute a modulation routine. The modulation routine receives the input signal corresponding to the angular position of the rotor and synchronizes a phase angle of the voltage output by the inverter to the angular position of the rotor.
These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood. however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.